|By Angus McDowall and Stephanie Nebehay

|By Angus McDowall and Stephanie Nebehay

BEIRUT/GENEVA (Reuters) - A Russian call for a five-hour truce on Tuesday failed to halt one of the most devastating campaigns of the Syrian war, where residents said government warplanes resumed striking the eastern Ghouta region after a brief lull.

Diplomatic sources meanwhile said the chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons, opened an investigation into attacks in eastern Ghouta to determine whether banned munitions were used.

Political leaders in France, the United States and Britain said this month they would back targeted military action against Damascus if there were proof chemical weapons had been used by forces under President Bashar al-Assad, Russia's ally.

Moscow and Damascus blamed rebels for the collapse of the truce, saying fighters had shelled a safe route intended for civilians to leave the enclave. The insurgents denied such shelling, and a senior U.S. general accused Moscow acting as "both arsonist and firefighter" by failing to rein in Assad.

Russian Foreign Minister Sergei Lavrov said Moscow would press on with a plan to stage similar daily pauses in the fighting, allowing aid to be delivered to eastern Ghouta through what Russia describes as a humanitarian corridor.

But the United Nations said it was proving impossible to aid civilians or evacuate wounded, and said all sides must instead abide by a 30-day truce sought by the U.N. Security Council.

"We have reports this morning there is continuous fighting in eastern Ghouta," U.N. humanitarian spokesman Jens Laerke said. "Clearly the situation on the ground is not such that convoys can go in or medical evacuations can go out."

Hundreds of people have died during 10 days of government bombardment of the eastern Ghouta, an area of towns and farms on the outskirts of Damascus. The assault has been among the most devastating air campaigns of a war now entering its eighth year.

The International Committee of the Red Cross (ICRC) said it was ready to access eastern Ghouta to deliver life-saving aid, but the proposed five-hour pause was too short.

Without mentioning Russia, the author of the proposal, ICRC Middle East director Robert Mardini said humanitarian corridors had to be well-planned and agreed to by all warring sides, while people should be allowed to leave of their own free will.

With its Ghouta offensive, the Syrian government is drawing on the military methods it has used to crush its opponents in other parts of Syria, including eastern Aleppo in late 2016.

"DEATH UNDER BOMBARDMENT"

Intensifying bombardment of the besieged area has been coupled with probing ground assaults to test rebel defenses.

With no sign of decisive international pressure to stop the attack, eastern Ghouta seems likely to meet the same fate as other areas won back by the government, where humanitarian corridors eventually became escape routes for defeated rebels.

"A concrete humanitarian corridor has been set up that will be used to deliver humanitarian aid, and, in the other direction, a medical evacuation can take place and all civilians who want to leave can," Lavrov told a joint news conference in Moscow after meeting French Foreign Minister Jean-Yves Le Drian.

But the local branch of the opposition Syrian Interim Government's Ministry of Health dismissed Russia's truce call as a ploy to circumvent the U.N. month-long ceasefire resolution.

The call in effect offered residents a choice between "death under bombardment" or a forced displacement, it said, calling on the United Nations to send in relief aid immediately.

U.S. Army General Joseph Votel accused Russia of playing a destabilizing role in Syria as "both arsonist and firefighter", saying Moscow had failed to rein in its Syrian ally.

"I think either Russia has to admit that it is not capable, or it doesn't want to play a role in ending the Syrian conflict. I think their role is incredibly destabilizing at this point."

Russia's military said rebels in eastern Ghouta started new offensives with intense artillery and gun fire after midday, Interfax news agency reported citing a Russian general.

ESCALATION

Residents in several towns in the eastern Ghouta described a brief pause in fighting, but said bombardment swiftly resumed. In Hammouriyeh town a man who identified himself as Mahmoud said helicopters and planes were in the sky and conducting strikes.

Siraj Mahmoud, a spokesman for the Civil Defence rescue service, which is funded by Western governments and operates in rebel areas, said artillery and air strikes had hit the region.

The Britain-based Syrian Observatory for Human Rights monitoring group said helicopters and warplanes had struck four towns and artillery shelling killed one person.

A U.N. Security Council resolution passed on Saturday called for a 30-day ceasefire across the entire country, but did not specify when it should start. It excludes some militant groups which are among the rebels in eastern Ghouta.

That has meant the ceasefire has not been observed in practice. U.N. spokesman Laerke declined to comment on the Russian proposal for a five-hour truce, but called instead on all sides to obey the full 30-day ceasefire.

"It is a question life and death ... we need a 30-day cessation of hostilities in Syria as the Security Council demands," Laerke, of the U.N. Office for the Coordination of Humanitarian Assistance, told a Geneva briefing.

A rebel spokesman said people in eastern Ghouta did not want to leave the area despite the bombardment, because they feared arrest, torture or conscription by the government. Russia said it would guarantee the safety of any civilians who left.

Eastern Ghouta, where the United Nations says around 400,000 people live, is a major target for Assad, whose forces have clawed back numerous areas with military backing from Russia and Iran.

Rebels based in eastern Ghouta have intensified shelling of government-held Damascus. A medical official in the capital said on Monday 36 people had been killed in four days. Syrian state media reported eight people hurt by rebel shelling on Tuesday. Damascus and Moscow say the campaign in eastern Ghouta is needed to halt such shelling.

The multi-sided Syrian war has killed hundreds of thousands of people and driven half of the pre-war population of 23 million from their homes. Fighting has escalated on several fronts this year, with the collapse of Islamic State giving rise to conflict between other Syrian and foreign parties.

As Assad has pressed the offensive against eastern Ghouta, Turkey has launched an incursion against Kurdish fighters in the northwestern Afrin region. Tensions have also flared between Iran and Israel, alarmed by Tehran's influence in Syria. Syrian air defenses shot down an Israeli F-16 earlier this month as it returned from a bombing raid on Iran-backed positions in Syria.

Diplomatic sources said the OPCW would examine attacks including one on Sunday which health authorities said killed a child and caused symptoms consistent with chlorine gas exposure.

Opposition-held areas in Eastern Ghouta were the scene of chemical attacks in 2013 in which hundreds of civilians were killed in the deadliest use of chemical weapons in decades.